A liquid crystal display device includes a pair of substrates and a liquid crystal layer sandwiched between these substrates. The substrate has a plurality of pixel electrodes regularly arranged in rows and columns (a matrix), and a driving voltage corresponding to an image signal is applied to each of the pixel electrodes. The optical characteristic (such as light transmittance or reflectance) of the liquid crystal layer is changed in each pixel through this voltage application, so that an image or a character can be displayed.
The method for applying an independent driving voltage to each pixel electrode on the substrate is divided into a “simple matrix method” and an “active matrix method”.
In the active matrix method, switching elements respectively corresponding to the pixel electrodes are arranged on the substrate. Such a substrate on which these switching elements are arranged is designated as an active matrix substrate. The switching element provided on the active matrix substrate performs switching of an electrical on/off state between the corresponding pixel electrode and a signal line. As such a switching element, a metal-insulator-metal (MIM) device or a thin film transistor (TFT) is suitably used.
The switching element is required to exhibit electric resistance as high as possible in an off-state. When strong light enters a switching element in an-off state, however, the electric resistance of the switching element is lowered so as to cause a leakage current, and hence, there arises a problem that charge stored in the corresponding pixel electrode is discharged. Also, a driving voltage at an appropriate level cannot be applied to the pixel electrode and a desired display operation cannot be executed, resulting in causing a problem that a contrast ratio is lowered because light leaks even in a dark state.
In the case where the liquid crystal display device is of transmission type, in order to solve the above-described problems, a mask layer designated as a black matrix is provided on the active matrix substrate or on a counter substrate opposing the active matrix substrate with the liquid crystal layer sandwiched therebetween. This black matrix reduces the area ratio (aperture ratio) of a pixel aperture. In order to attain high refinement by reducing the area occupied by the black matrix, the switching elements and lines are reduced, but when the switching elements and the lines are reduced, the driving power is lowered and the line resistance is increased disadvantageously. Also, due to restriction of fabrication technology, the switching elements and the lines are difficult to reduce.
For the purpose of attaining high refinement by utilizing a non-display region on a black matrix, U.S. Pat. No. 4,984,091 discloses a technique in which a displayed image is optically shifted by a distance substantially equivalent to a pixel pitch. According to this technique, a corresponding image is displayed in shifted positions of pixels in synchronization with the shift of the pixels. As a result, the apparent number of pixels is increased, and hence, even when a display device with low resolution is used, display similar to that attained by using a highly refined display panel can be produced.
U.S. Pat. No. 6,061,103 discloses a method in which red, green and blue (hereinafter together referred to as the “RGB”) pixels are optically successively shifted by using a shifting device so as to simultaneously display shifted pixels. In this method, in a region corresponding to one pixel, the RGB pixels are displayed in a time division manner. As a result, without reducing the pixel pitch on the display panel, the apparent resolution can be increased three times.
Also, U.S. Pat. No. 6,061,103 discloses, as means for optically shifting an image, an image shifting device composed of a combination of a liquid crystal device and a birefringent device. The birefringent device is made from a material in which a refraction direction of light is changed depending upon the polarization direction of the incident light. When the polarization direction of light entering the birefringent device is changed by using the liquid crystal device, the optical axis of light outgoing from the birefringent device can be shifted.
FIG. 1 shows a known image shifting device. This image shifting device includes a liquid crystal device 10 and a birefringent device 11 arranged in a line along a light propagation direction. The liquid crystal device 10 switches the polarization state of incident linearly polarized light between a state where the plane of the vibration of the electric field vector (hereinafter referred to as the “polarization plane”) of the linearly polarized light is rotated by 90° and a state where the linearly polarized light is allowed to pass therethrough without rotating the polarization plate. The birefringent device 11 can shift the light in accordance with the direction of the polarization plane of the incident linearly polarized light.
In the image shifting device exemplified in FIG. 1, the electric field vector direction (polarization direction) of light entering the liquid crystal device 10 is vertical to the surface of the drawing. Since the liquid crystal device 10 uses liquid crystal of the TN mode (TN liquid crystal) with positive refractive index anisotropy Δ∈, liquid crystal molecules are twisted by 90° when no voltage is applied through the liquid crystal layer of the liquid crystal device 10 (namely, in a voltage off-state), and therefore, the polarization plane of the incident light is rotated by 90° owing to the optical activity. On the other hand, when a voltage of a predetermined or higher level is applied through the liquid crystal layer of the liquid crystal device 10 (namely, in a voltage on-state), the orientation of the liquid crystal molecules accord with the direction of the electric field, and therefore, the incident light outgoes with the polarization plane vertical to the surface of the drawing. The birefringent device 11 of this drawing allows light with the polarization plane vertical to the surface of the drawing to pass therethrough but can shift light with the polarization plane parallel to the surface of the drawing.
The liquid crystal device 10 included in the image shifting device shown in FIG. 1 is required to appropriately and rapidly switch, in accordance with the magnitude of the applied voltage, the state of outgoing light between a state for allowing first linearly polarized light to outgo and a state for allowing second linearly polarized light having a polarization plane vertical to that of the first linearly polarized light to outgo.
As described above, in the case where the liquid crystal device uses the TN liquid crystal, the linearly polarized light having entered the liquid crystal device is allowed to outgo as the linearly polarized light with the polarization plane rotated by 90° when no voltage is applied through the TN liquid crystal. However, when a voltage is applied through the TN liquid crystal, the orientation of the liquid crystal molecules is rapidly changed by the electric field, and hence the liquid crystal is transited to the state where the polarization of the incident light is not changed. On the other hand, when the voltage application through the TN liquid crystal is stopped, the liquid crystal molecules are transited (relaxed) to the initial state but merely slowly.
In this manner, the speeds of changing the orientation of the liquid crystal molecules are different between the case where the voltage applied through the liquid crystal layer is changed from Low level (typically, of 0 V) to High level (of, for example, 5 V) and the case where the applied voltage is changed from High level to Low level. In order to evaluate these response speeds, a pair of orthogonal polarizers are disposed in front of and behind the liquid crystal layer so as to measure change with time of light transmittance. FIG. 2 shows the change of transmittance obtained when the applied voltage is changed from Low level to High level and then changed from High level to Low level after a predetermined time has elapsed. At this point, time elapsed from the transmittance lowering from the maximum value to zero is designated as “liquid crystal rise response time τr” and time elapsed from the transmittance increasing from zero to the maximum value is designated as “liquid crystal fall response time τd”. The liquid crystal rise response time τr is comparatively short but the fall response time τd is comparatively long. When the liquid crystal fall response time τd is long, an image cannot be shifted in synchronization with switching timing of an image to be displayed by the image display device. Before describing this problem, an image switching speed of an image display device will be first described.
In general, as a driving method for an image display device, either interlace driving or noninterlace driving is employed. The interlace driving is a display method in which odd lines alone and even lines alone are respectively selected in one field so as to complete one image by using the odd and even fields, and selection time for each field is generally 16.6 milliseconds (60 Hz). On the other hand, the noninterlace driving is a display method in which lines are successively selected regardless of odd lines and even lines of the display device, and selection time for each field is generally 16.6 milliseconds (60 Hz) in the same manner as in the interlace driving. At this point, a field means a period of a vertical portion of an image in either of the interlace driving and the noninterlace driving. In a liquid crystal display device, a scan period including blanking time corresponds to a field period.
In the method described in the aforementioned U.S. Pat. No. 6,061,103, one field period is divided in accordance with the shifted positions of RGB pixels, and different images (“sub-field images”) are displayed by the image display device in the respective divided periods (hereinafter referred to as the “sub-field periods”). The sub-field period is approximately 5 milliseconds in this case, and therefore, the image shifting device needs to shift the image at short time intervals of approximately 5 milliseconds. Furthermore, an image should be shifted by the image shifting device in synchronization with switching timing of the sub-fields, and therefore, the image shifting device is required to transit the state in response to the voltage applied to the liquid crystal device simultaneously with the switching of the sub-fields.
In an actual liquid crystal device, however, it is difficult to rapidly transit the state in response to the voltage application. For example, in the TN liquid crystal, as shown in FIG. 2, the rise response time τr is comparatively short but the fall response time τd is generally several tens milliseconds, which is longer than the selection time for a sub-field.
Such a difference in the response time is caused because the “rise” of the curve shown in FIG. 2 is caused by forcedly orienting the liquid crystal molecules along one direction by applying the voltage through the liquid crystal while the “fall” thereof is caused by naturally relaxing the orientation of the liquid crystal molecules to the initial state by stopping the voltage application through the liquid crystal.
When such liquid crystal having the long fall response time τd is used, there arises a problem that polarization cannot be appropriately switched. Referring to FIG. 1, this problem will be described. As shown in FIG. 1, when the voltage application to the liquid crystal device (liquid crystal cell) 10 is changed from an on-state to an off-state, the polarization plane of the light outgoing from the liquid crystal device 10 is rotated by 90°, and as a result, the optical axis of the light outgoing from the birefringent device 11 is shifted from a position B to a position A. At this point, if the fall response time τd is too long, the linearly polarized light is changed into elliptically polarized light at a transient stage of the fall, and therefore, the same images are doubly displayed in both the position A and the position B, resulting in lowering the resolution.
Furthermore, when there is a large difference between the fall response time τd and the rise response time τr, a difference is caused in the generation level of the double image between the shift from the position A to the position B and the reverse shift, and this difference is visually recognized as a flicker.
Japanese Laid-Open Patent Publication No. 2000-199901 describes that, in a TN liquid crystal display apparatus, the liquid crystal response speed attained when the voltage is changed from an on-state to an off-state can be improved by increasing a twist angle peculiar to the liquid crystal by adjusting the concentration of a chiral agent added to the liquid crystal. However, in a liquid crystal display apparatus, when the concentration of the chiral agent is thus increased, the amplitude of the voltage to be applied through the liquid crystal layer in a voltage on-state should be increased than in a conventional technique, but this is practically difficult in consideration of the performance of a semiconductor thin film transistor working as a switching element in a display region. Furthermore, there is another problem that as the twist angle peculiar to the liquid crystal is increased, the state of the liquid crystal attained in a voltage off-state is more unstable, and hence, the twist angle of 90° can be kept for merely a short period of time. Accordingly, the aforementioned technique has not been applied to an actual liquid crystal device.
Moreover, in a liquid crystal display device, when a distance between substrates is varied on a display plane due to deformation of the substrates caused by an external pressure or the like, a threshold voltage is varied, a short-circuit is caused between electrodes of the respective substrates or the orientation of liquid crystal molecules are disturbed, resulting in a problem that good display cannot be produced. Therefore, a method in which spacers (supporting bodies) for keeping constant a distance between the pair of substrates (which is also designated as a cell thickness or a cell gap) are provided between the substrates has been proposed.
As a method for providing spacers between substrates, for example, an organic or inorganic film is formed on a substrate, a resist is applied thereon, and the resist is subjected to mask exposure, development and etching, so as to form the spacers. Alternatively, instead of the photoresist, a photosensitive organic resin such as photosensitive polyimide or photosensitive acrylic resin may be used.
According to this method, a spacer can be formed in an arbitrary place (such as a place outside a pixel region) and a contact face between the substrate and the spacer can be formed in an arbitrary pattern, and hence, this method is good in view of uniformity in the cell thickness, the strength against the external pressure and the display quality. In this method, procedures for forming spacers, forming an alignment film (an orientation controlling film) and a uniaxial orientation treatment (such as a rubbing treatment) are performed in the order of, for example, any of the following orders (a) through (c):
(a) An alignment film is first formed on a substrate, the alignment film is then subjected to the uniaxial orientation treatment and thereafter, spacers are formed on the alignment film.
(b) An alignment film is first formed on a substrate, spacers are then formed on the alignment film and thereafter, the alignment film is subjected to the uniaxial orientation treatment.
(c) Spacers are first formed on a substrate, an alignment film is then formed and thereafter, the alignment film is subjected to the uniaxial orientation treatment.
Among the above-described orders (a) through (c), the order (c) in which the alignment film is formed after forming the spacers is more preferable than the orders (a) and (c) in which the spacers are formed on the alignment film. This is because, in the order (a) or (b), when the spacers are formed by using, for example, a photoresist or a photosensitive organic resin, a diluting solvent, a developer and a repellent used in applying such a material may lower the orientation-regulating force of the alignment film, and hence, it is difficult to attain high display quality.
In the aforementioned method, a spacer can be formed in an arbitrary shape in an arbitrary position, and therefore, as compared with the case where bead spacers are spread, display nonuniformity derived from spread nonuniformity can be avoided. Furthermore, when a spacer is disposed to overlap a mask layer (black matrix) provided in a position outside pixels, lowering of the display quality derived from display of the spacer itself can be prevented. However, since the orientation of the liquid crystal molecules is disturbed in the vicinity of the spacer, light leakage is caused due to an orientation defect region in the vicinity of the spacer, resulting in lowering the contrast ratio.
The lowering of the display quality is particularly serious when the liquid crystal display device is used in a projection image display apparatus. This is because, in a projection image display apparatus, light emitted from a light source and passing through the liquid crystal display device is enlarged and projected on a screen.
As a method for solving the aforementioned problem, a method using a mask layer for covering the spacer itself and a region in the vicinity of the spacer where the light leakage is caused has been proposed. In this method, the region where the light leakage is caused due to the spacer is covered with the mask layer, and therefore, the lowering of the contrast ratio is suppressed.
Also, Japanese Laid-Open Patent Publication No. 2001-109005 discloses a method in which a column-shaped spacer itself is provided with uniaxial orientation, so as to suppress the orientation turbulence in the vicinity of the spacer.
However, when the mask layer for covering the region where the light leakage is caused is formed, the aperture ratio is lowered, and hence, it is disadvantageously difficult to produce bright display.
Furthermore, the method disclosed in Japanese Laid-Open Patent Publication No. 2001-109005 is difficult to employ for mono domain liquid crystal orientation typically of the TN mode. In this method, the orientation of the liquid crystal molecules in the vicinity of the spacer is fixed by the spacer. Therefore, in a liquid crystal display apparatus for producing display in, for example, the TN mode, the liquid crystal molecules are oriented concentrically around the spacer, and some liquid crystal molecules are inclined against the polarization axis of a polarizing plate. This causes light leakage. In order to suppress the occurrence of the light leakage, it is necessary to orient the liquid crystal molecules parallel or orthogonal to the polarization axis, and what is called four-domain orientation should be employed.
Furthermore, a TN liquid crystal display device has a problem that a response speed Doff attained in turning the voltage off is low. In general, with respect to the response speed τoff attained in turning the voltage off, the following approximate equation (1) is widely known, wherein the viscosity of the liquid crystal is indicated by η, the thickness of the liquid crystal layer is indicated by d and the elastic coefficient of the liquid crystal is indicated by K:τoff=η·d2/(τ2K)  (1)
On the basis of this equation, it can be presumed that it is preferred to lower the viscosity η of the liquid crystal or reduce the thickness d of the liquid crystal layer in order to improve the response speed τoff attained in turning the voltage off. Conventionally, a variety of improvements in development of a liquid crystal material with low viscosity or reduction of the thickness of a liquid crystal layer have been examined from this point of view.
Moreover, differently from the above-described method, Japanese Laid-Open Patent Publication No. 2000-199901 discloses that liquid crystal is previously strongly twisted and is placed in a twisted state as a standard state, namely, as a state attained in a power off-state, and further describes that when the voltage is turned off, the liquid crystal positively recovers by its own force from a non-twisted state attained in a power on-state to the twisted state, whereby largely improving the response time attained in turning the voltage off. Also, it describes that the liquid crystal itself rapidly recovers the twisted structure due to the twist peculiar to the material of the liquid crystal after turning off the voltage, and therefore, a back flow can be suppressed, so as to increase the response speed.
The conventional improvement of the response speed attained in turning the voltage off has, however, the following problems:
First, when the response speed attained in turning the voltage off is to be improved on the basis of the equation (1), the viscosity η of the liquid crystal is reduced. However, the liquid crystal has physical property values peculiar to the material of the liquid crystal, such as the viscosity, the elastic coefficient, the dielectric constant, the refractive index and the phase transition temperature, and these physical property values are correlated with one another. Therefore, it is difficult to reduce the viscosity alone of the liquid crystal, and hence, this method cannot attain a sufficient effect.
Furthermore, when the thickness d of the liquid crystal layer is reduced, the improvement in the response speed attained in turning the voltage off can be expected, but the retardation R (R=d·Δn, wherein Δn is the refractive index anisotropy of the liquid crystal) of the liquid crystal layer is reduced, resulting in lowering the light transmittance.
Moreover, when the elastic coefficient K of the liquid crystal is increased, the improvement in the response speed attained in turning the voltage off can be expected, but the elastic coefficient K of the liquid crystal depends upon the chemical structure of the liquid crystal molecule, and hence, it is substantially impossible to set the elastic coefficient K of the liquid crystal independently to a desired value.
In this manner, the improvement in the response speed attained in turning the voltage off on the basis of the equation (1) cannot attain a sufficient effect because of difficulty in improvement of the material with respect to the liquid crystal itself and because of the trade-off relationship between the response speed and the light transmittance with respect to the thickness of the liquid crystal layer.
Also, in the method for improving the response speed attained in turning the voltage off by increasing the twist angle peculiar to the material of the liquid crystal, namely, by reducing the twist pitch peculiar to the material of the liquid crystal, as disclosed in Japanese Laid-Open Patent Publication No. 2000-199901, change of the light transmittance attained under voltage application tends to be rather slow as described in the publication. This tendency is conventionally widely known, and for example, Japanese Laid-Open Patent Publication No. 5-181165 describes that when the twist pitch peculiar to the material of liquid crystal is reduced by increasing the amount of a chiral material to be added, it is necessary to increase the driving voltage for increasing the contrast. Also, Japanese Laid-Open Patent Publication No. 4-278929 has similar description. In this manner, in the method for improving the response speed attained in turning the voltage off by reducing the twist pitch peculiar to the material of liquid crystal, the driving voltage should be increased, and hence, this method is practically difficult to employ in consideration of the performance of a semiconductor transistor or the like working as a switching element.